The present invention relates to an image forming apparatus that conducts an image forming operation based on the electro-photographic process so as to form a toner image onto a circumferential surface of a rotating photoreceptor drum, and specifically relates to a rotation driving control operation for controlling the rotating velocity of the photoreceptor drum so as to keep it constant.
In the image forming apparatus employing the electro-photographic process, an image is formed on a recording medium in such a manner that a toner image is formed on a image bearing member, such as the rotating photoreceptor drum, a photoreceptor belt, etc., and then, the toner image formed on the image bearing member is directly or indirectly transferred onto the recording medium, and further, the toner image residing on the recording medium is fixed thereon.
In the event of performing the abovementioned image forming operation, if the velocity of the image bearing member, which is to be rotated at a constant velocity when a latent image is formed by the image exposing operation conducted by an exposing device, fluctuates, distortions in the sub-scanning direction are generated on the image to be formed.
Further, in a color image forming apparatus employing a tandem image forming method, a full color toner image is formed on a recording paper sheet by superimposing unicolor toner images, which are formed by a plurality of unicolor toner-image forming units, respectively. Accordingly, it has been an indispensable condition for acquiring a good color image that the image bearing members provided in the plurality of unicolor toner-image forming units should be rotated at the same velocity without generating any velocity unevenness. Therefore, if the velocities of the image bearing members provided in the plurality of unicolor toner-image forming units are different from each other, color deviations would be generated.
So far, with respect to the method for controlling the velocity of the photoreceptor drum, there has been proposed various kinds of controlling methods. Among other thing, when trying to control the angular velocity of the photoreceptor drum in real time mode so as to keep it at a constant velocity, there has been employed the rotation velocity controlling method for conducting the rotation velocity controlling operation by using the angular velocity detecting element, such as an encoder.
In this connection, with respect to the above-kind of rotation velocity controlling operation, the proposals for conducting efficient processing are set forth in Tokkaihei 6-327278 and Tokkai 2003-186368 (both are Japanese Patent Application Laid-Open Publications).
With respect to the angular velocity controlling operation, the above-cited Tokkalhei 6-327278 sets forth such a proposal that the profile of the rotation unevenness of the photoreceptor drum concerned is measured at a certain time point by using the feedback control or the feed forward control so as to employ the data of the profile for the angular velocity controlling operation until the predetermined next sampling time will arrive.
Further, the above-cited Tokkai 2003-186368 sets forth such a proposal that, when the angular velocity controlling operation of the photoreceptor drum is performed by employing both the feedback control and the feed forward control, the driving operation at the present drum position is performed by referring the deviation value of the angular velocity derived from that of one revolution before, while continuously updating the angular velocity profile stored in the memory, so as to abruptly converge the unevenness (fluctuation) of the angular velocity onto the constant value.
Incidentally, it is performed that the residual toner remaining on the circumferential surface of the photoreceptor drum is removed by press-contacting the cleaning blade onto the photoreceptor drum. On that occasion, the abrasive load generated between the circumferential surface of the photoreceptor drum and the cleaning blade is greatly fluctuated by various kinds of factors, such as environmental conditions, an image forming history, a characteristic aging variation of the cleaning blade, etc. Accordingly, the feedback controlling method is employed so as to rotate the photoreceptor drum at the predetermined constant velocity, even if such the fluctuation of the abrasive load is generated.
The present inventors have revealed that, when the fluctuation of the abrasive load is generated as abovementioned, a certain inconvenience occurs in the rotation velocity controlling operation that employs the feed forward control method. This inconvenience will be detailed in the following.
FIG. 5a shows a graph indicating a transition of the rotation velocity exhibiting a repeatable fluctuation that occurs every one revolution of the photoreceptor drum. In this connection, the broken line indicates a command velocity instructed by the controlling section. Further, in order to make the explanation simple, fine velocity fluctuations and other velocity fluctuations having no repeatability are omitted from the graphs.
With respect to such the velocity fluctuation having the repeatability, in order to generate a waveform having a phase being reverse to that of the waveform indicated in FIG. 5a, taking the phase delay component into account, the instruction value waveform, having a phase that is slightly advanced from the reverse phase, is created as shown in FIG. 5b. Successively, based on the above-mentioned instruction value waveform, the waveform having the reverse phase is generated as shown in FIG. 5c. Still successively, by employing the above-generated waveform as the instruction value for canceling the velocity fluctuation, it becomes possible to acquire the constant rotation velocity of the photoreceptor drum as shown in FIG. 5d, indicating a state that the velocity fluctuation is cancelled.
FIG. 6a shows a graph indicating a transition of the rotation velocity exhibiting a repeatable fluctuation that occurs every one revolution of the photoreceptor drum. In this connection, the broken line indicates a command velocity instructed by the controlling section. Further, in order to make the explanation simple, fine velocity fluctuations and other velocity fluctuations having no repeatability are omitted from the graphs.
With respect to such the velocity fluctuation having the repeatability, in order to generate a waveform having a phase being reverse to that of the waveform indicated in FIG. 6a, taking the phase delay component into account, the instruction value waveform, having a phase that is slightly advanced from the reverse phase, is created as shown in FIG. 6b. 
Successively, based on the abovementioned instruction value waveform, the waveform having the reverse phase, indicated by the bold broken line c1 shown in FIG. 6c, is scheduled to be generated. In this connection, when the load for the photoreceptor drum reduces, the toque of the driving motor for driving the photoreceptor drum is also getting small. For this reason, based on the instruction value, shown in FIG. 6b, which is found in the state that the normal toque is assumed, the instruction value of the reverse waveform is getting large relative to the lowered toque, as indicated by the bold solid line c2 shown in FIG. 6c, and as a result, the reverse waveform, having an amplitude being larger than the scheduled amplitude, is generated. Still successively, by employing the above-generated reverse waveform as the instruction value for canceling the velocity fluctuation, the rotation velocity of the photoreceptor drum as shown in FIG. 6d, indicating a state that the rotation velocity is greatly fluctuated in a direction reverse to that of the original velocity fluctuation, could be obtained.
In other words, when the fluctuation of the load is generated in the photoreceptor drum, the feed forward controlling operation is not appropriately performed. Further, in this case, the feed forward controlling system may oscillate, and as a result, sometimes, the driving motor would be stopped by the abnormal drive malfunction caused by the vibrations, etc.
Further, in order to prevent the occurrence of the abovementioned inconvenience, such as the oscillation, etc., it may be possible to determine parameters to be used for the feed forward controlling operation, in a state that gain of the system is made to reduce in advance. However, in that case, although no malfunction, such as the oscillation, etc., is generated, the effect of the feed forward controlling operation becomes weak. Accordingly, there have arisen new problems that the effect of suppressing the periodical velocity fluctuation is small, and/or the suppressing operation consumes much time or the like.